Need to experiment to see how wide the area covered by the distance sensor is for getting the z-coordinate value without compromising the x/y-coordinates. And how lighting needs to be, so that shadows don’t cause a problem.

Email Ben re. proceeding, and borrowing Arduino for experimentation. Get web cam, and try to figure out how to get x- and y- coordinates out of it, in a reasonably reliable fashion. Get skates on asap, there is lots of work to do.